The present invention relates to an image display apparatus, and more particularly, to a light emission type image display apparatus suitable for displaying an image using current driven display elements, specifically, organic light emitting diodes (LED).
An organic EL-based flat image display apparatus has been known as one type of image display apparatus. This type of image display apparatus employs a driving method using low temperature polysilicon TFTs (thin film transistors) in order to implement a high luminance active matrix display, for example, as described in SID 99 technical digest, pages 372–375. For employing this driving method, the image display apparatus takes a pixel structure in which scanning wires, signal wires, EL power supply wires and capacitance reference voltage wires are intersected with one another, and has a signal voltage holding circuit formed of an n-type scanning TFT and a storage capacitor for driving each EL. A signal voltage held in the holding circuit is applied to a gate of a p-channel driving TFT arranged in a pixel to control the conductance of a main circuit of the driving TFT, i.e., the resistance value between its source and drain. In this structure, the main circuit of the driving TFT and an organic EL element are connected in series with each other from an EL power supply wire, and also connected to an LED common wire.
For driving a pixel configured as described above, a pixel selection pulse is applied from an associated scanning wire to write a signal voltage into the storage capacitor through a scanning TFT for holding the signal voltage. The held signal voltage is applied to the driving TFT as a gate voltage to control a drain current in accordance with the conductance of the driving TFT determined from a source voltage connected to a power supply wire, and a drain voltage. As a result, a driving current of the EL element is controlled to control a display luminance. In this event, in the pixel, a source electrode of the driving transistor is connected to the power supply wire, which causes a voltage drop. The driving transistor has a drain electrode connected to one end of the organic LED element, the other end of which is connected to a common electrode shared by all pixels. The driving transistor is applied with the signal voltage at its gate, such that the operating point of the transistor is controlled by a differential voltage between the signal voltage and source voltage to realize a gradation display.
However, when the foregoing configuration is applied to implement a large-sized panel, voltages for driving pixels in a central region of the panel are lower than voltages for driving pixels in a peripheral region of the panel. Specifically, the organic LED element is current driven, so that if a current is supplied to a pixel in a central region of the panel from a power supply through a LED common wire, a voltage drop is caused by the wire resistance, thereby reducing the voltage for driving the pixel in the central region of the panel. Since this voltage drop is affected by the length of the wire and a display state of pixels connected to the wire, the voltage drop also varies depending on displayed contents.
Further, the operating point of a driving transistor for a pixel largely varies in response to a varying source voltage of the driving transistor connected to the LED common wire, so that a current for driving LEDs largely varies. The variations in current cause variations in the luminance of display, i.e., uneven display and non-uniform luminance, as well as cause a defective display in the form of non-uniform color balance in the screen when a color display is concerned.
To solve these problems, JP-A-2001-100655, for example, has proposed an improvement on a voltage drop caused by a wire by reducing a wiring resistance. In a system described in JP-A-2001-100655, a conductive light shielding film having an opening for each pixel is disposed over the entire surface of a panel and connected to a common power supply wire to reduce the wire resistance and accordingly improve the uniformity of display.
However, in the system described in JP-A-2001-100655, since a source electrode, acting as a reference voltage for a transistor for driving an organic LED in a pixel is connected to an LED common electrode shared by the panel, some voltage drop is produced between the source electrode and common electrode. For this reason, even if the same signal voltage is applied, the gate-source voltage, which determines the operating point of the transistor, varies in response to variations in the source voltage, thereby encountering difficulties in removing the non-uniformity of display.
Also, the foregoing system has such a nature that variations in a threshold value, i.e., the on-resistance of a driving TFT for driving an EL cause a change in an EL driving current even if the same signal voltage is applied for controlling the current, so that TFTs which exhibit few variations and uniform characteristics are required for implementing the system. However, transistors for use in realizing such a driving circuit are obliged to be low temperature polysilicon TFTs which are manufactured using a laser anneal process and are high in mobility and applicable to a large-sized substrate. However, the low temperature polysilicon TFTs are known to suffer quite a few variations in element characteristics. Thus, due to the variations in the characteristics of TFTs used in an organic EL driving circuit, the luminance varies pixel by pixel, even if the same signal voltage is applied, so that the low temperature polysilicon TFT is not suitable for displaying a highly accurate gradation image.
As a driving method for solving the foregoing problems, JP-A-10-232649, for example, proposes a driving method for providing a gradation display which divides a one-frame time into eight sub-frames which are different in display time, and changes a light emitting time within the one-frame time to control an average luminance. This driving method drives a pixel to display digital binary values representing a lit and an unlit state to eliminate the need for using the operating point near a threshold value at which variations in the characteristics of TFTs are notably reflected to a display, thereby making it possible to reduce variations in luminance.
Any of the foregoing prior art techniques does not sufficiently consider the non-uniformity in luminance due to a voltage drop on a power supply wire of organic LEDs, and fails to solve a degraded image quality due to the voltage drop on the power supply wire, particularly in a large-sized panel.
In addition, the prior art techniques may reduce the conductance of the transistors to set a high LED power supply voltage for preventing a varying voltage on the LED common wire, thereby reducing variations in luminance. However, this leads to a lower power efficiency and increased power consumption of a resulting image display apparatus. Also, since a transistor presenting a low conductance has a longer gate length, the transistor has a larger size which is a disadvantage in regard to the trend of higher definition.